Various types of pacing leads have been developed for endocardial implant, typically in the right ventricle (RV) or right atrial appendage, as well as the coronary sinus. These flexible leads usually are constructed having an outer polymeric sheath encasing one or more electrical conductors. One conductor is typically attached at its distal tip to the shank portion of a tip electrode. In bipolar or multipolar leads, one or more further conductors are provided in coaxial or co-linear relation to the first conductor and is connected at its distal end to a more proximally located, ring-shaped electrodes situated along the lead body. The proximal ends of each conductor are coupled to a connector, which includes a single pin in unipolar leads, and additional pins or in-line rings in bipolar and multipolar leads.
Stimulation in both single chamber and dual chamber pacemakers with ventricular stimulation normally takes place in the apex of the RV. A conventional pacemaker of this type typically requires two electrodes. One of these electrodes is placed in the right atrium (RA) and the other in the apex of the RV. The electrodes sense electrical activity in the heart and to provide stimulation pulses as needed to rectify arrhythmias.
Further, while stimulation in the apex has proved clinically effective, there is a need to limit RV pacing to more closely simulate the natural cardiac system.
In a healthy heart, electrical potential originates in the sinoatrial (SA) node, travels to the atrioventricular (AV) node, and finally to the myocardial mass through the Purkinge fibers. This provides a sequential activation of the atria and the ventricles. Specifically, the sequential polarization and depolarization of the atria and the ventricle results in a naturally synchronized sinus rhythm.
There is increasing evidence that the sequence of electrical activation is necessary for the normal functioning of the heart. Presently there are three major characteristics of proper electrical activation of a heart: (1) left ventricle (LV) activation before RV activation, (2) in the LV, endocardial activation before epicardial activation, and (3) in both the RV and LV, apex activation before base activation. The earliest electrical activation typically occurs at the endocardium of the lower left side of the septum and the lower anterior wall.
Recent experiments have shown that creation of an activation sequence similar to that of the natural contraction of the heart contributed to better heart functioning. Patients with poor atrio-ventricular conduction (AV-block) or poor sinus-node function typically receive a ventricular pacemaker. Such pacemaker restores the normal heart operation. However, the traditional position of the ventricular lead is the right ventricular apex. This pacing location may not provide optimal heart functioning and may result in ventricular remodeling.
Further, use of ventricular pacing is to resynchronize the ventricular activation. This is mainly used in patients with heart failure (HF) who also have left bundle branch block (LBBB). LBBB causes a sequence of activation similar to that during RV apex pacing. In the LBBB patients, left ventricular (LV) or biventricular pacing (BiV) is used to resynchronize ventricular activation, with good results.
Although LV pacing sites may offer great advantages over RV pacing sites, positioning of the leads is cumbersome. For example, thoracotomy is used to position a lead at the LV wall, but this invasive procedure does not offer significant advantages over the transvenous approach. In the transvenous approach, LV pacing leads are positioned in coronary veins, which can be difficult, time consuming, and not reliable because of lead displacement. Moreover, even if the lead can be positioned in a coronary vein, individual heart anatomies differ widely, such that the preferred site could be difficult to locate for lead replacement.
Recently a trans-atrial septal approach was introduced to enable LV endocardial pacing. In this approach the pacing lead is advanced from the right atrium through the right atrial septum wall into the left atrium and through the mitral valve into the LV cavity until a proper site at the LV endocardium is reached. A disadvantage with this approach is that a large part of the lead is permanently inside of the LV cavity. One of the many clinical concerns with the trans-atrial septal approach is Emboli originating from this lead may enter the systemic circulation giving rise to strokes.